Phosphorus and Water Flux Dynamics in Runoff and Plant Uptake in Forested Headwaters (since 2017, funded by German Research Foundation, DFG)

Description: Forest stands are known to be affected by shortage in phosphorus, an important nutrient for plants. The main source of phosphorus in forested ecosystems is either recycling of litter by microbial activity in the shallow soil layers (recycling system) or weathering of the parent material in deeper depth of the soil profile (acquiring system). The project investigates the role of subsurface runoff processes that impact the processes of nutrient supply by export of phosphorus from forest stands. Results from the first project phase indicate, that a large fraction of plant available P is lost from the forest stand by deep percolation. To better quantify this loss and to study the recovery of phosphorus supply at various soil depth after a rainstorm, hydrometric and hydrochemical monitoring at three experimental hillslopes in Germany is planned. The instrumental setup consists of a 2 m deep trench, soil moisture measurements at 5 different depth, a > 15 m deep groundwater well and spring- and streamflow measurements at the bottom of the hillslope and will be extended by a horizontal drainage that allows sampling of percolation in deeper soil depth below the trench. This experimental setup will allow quantifying the vertical re-distribution of water and nutrients during controlled sprinkling experiments and the following period of recovery of phosphorus. In addition, sap flow of trees will be monitored and sampled to better understand and distinguish the sources and coupling of water and nutrient supply of plants in recycling and acquiring ecosystems. The project aims at a better characterization of differences in susceptibility of recycling and acquiring ecosystems to an anticipated increase in nutrient flushing frequency under changed climatic conditions. The findings are planned to be generalized, integrated and tested in a model that allows to simulate ecosystem nutrition strategies in forest stands.

Description: The project builds upon previous work on topographic controls on catchment-scale groundwater dynamics (Rinderer et al., 2014, Rinderer et al., 2016) in deriving spatial patterns of groundwater response under consideration of the catchment physiographic characteristics, rainfall characteristics and antecedent soil saturation using time series clustering (Rinderer et al., 2017). Distance-based similarity measures (the integral of two time series) captured similarity in the shape and amplitude of the groundwater time series and were correlated with topographic indices. Correlation-based similarity measures (based on the cross-correlation of two time series) were more sensitive to seasonal differences in groundwater dynamics. Based on these results it was possible to transfer groundwater dynamics from more than 50 monitoring sites to all non-monitored locations of a Swiss pre-alpine catchment. The groundwater dynamics of midslope locations were found to be most difficult to predict but had already been highlighted in earlier studies to be of key-importance in establishing hydrological connectivity between uphill sites and the lower hillslopes and streams, respectively (Rinderer et al., 2017). An analysis of 19 months-long time series showed that the area with groundwater response that also was connected to the stream and therefore likely contributed to runoff, expanded and contracted predominantly aligned with the channel network. Isolated areas with groundwater response got connected to the stream network during short periods of time that were associated with highest streamflow (Rinderer et al., in preparation). Clear exponential relations between specific discharge and connected area or total catchment storage were identified that were very similar for the rising and falling limb of event hydrographs and for events with multiple peaks (Rinderer et al., in preparation). The results of this project show the importance of hydrological connectivity to better understand the often non-liner runoff response in headwater streams.

Soft Soil Moisture Sensing – Africa

Qualitative soil moisture assessment in semi-arid Africa - The role of experience and training on inter-rater reliability (2014, funded by the Swiss Agency for Development and Cooperation - DEZA)

Description: Soil moisture differences are important indicators of the soil water deficit and have traditionally been used for allocating water resources among farmers of village communities in rural Africa. We adapted the Boots & Trousers Method (Rinderer et al., 2012) to incorporate the local farmers’ tacit knowledge and the soil properties of the study site near Arusha, Tanzania. The qualitative indicators to distinguish between the seven wetness classes are related to moisture conditions for seeding plants and brick making in this semi-arid environment. The scheme was tested twice in 2014 with farmers, students and experts (April: 40 persons, June: 25 persons) for inter-rater reliability, bias of individuals and the importance of training how to apply the method. During the test in June in 66% of all classifications farmers assigned the same wetness class and were off by not more than one wetness class in 90%. However it was important to organize the group in small sub-groups and to provide them with good instructions. Students who had been trained in how to apply the method gained higher inter-rater reliability than their colleagues with only a basic introduction. This study demonstrates that a wetness classification scheme based on qualitative indicators can be a robust tool for capturing soil moisture variability.